Every foreign criminal jailed for at least six months must face automatic deportation to end the prison overcrowding crisis, ministers have been told.
Revealing a series of proposed amendments to Labour's hugely controversial Sentencing Bill, Reform's Sarah Pochin said the Government must scrap plans to let convicts out early.
The former magistrate warned public safety will be put at risk under the current plans.
Ms Pochin urged ministers to automatically deport anyone convicted to longer than six months in prison.
And Reform are demanding that, if the plans do go ahead, the Government commits to regularly reviewing the sentencing reforms' impact.
Nigel Farage's party also wants Clause 1 of the Sentencing Bill - which introduces a presumption to suspend short custodial sentences of 12 months or less - to be scrapped amid fears it could lead to an increase in knife crime.
Party officials, explaining the proposed change, said: "Questions remain as to what this means for those found carrying knives.
"The current position is that anyone found in possession of a bladed article receives a mandatory six months custodial sentence. Is the Secretary of State for Justice really now suggesting in this Bill that those offenders have that custodial sentence suspended?"
Reform UK's Sarah Pochin MP said: "This Bill is motivated by the desire to free up prison spaces and not by what is the best way to administer justice. Its primary function is not the protection of the public, or the protection of victims of crime or abuse, in fact quite the opposite.
"A Reform government will prioritise keeping the public safe, protecting women and children and have zero tolerance to knife crime. If we need more prison spaces to keep dangerous criminals off our streets then Reform will build more prisons. It's that simple."
Around 12% of the prison population are foreign nationals, Ministry of Justice records show.
Keeping a foreign prisoner behind bars in the UK costs £54,000 a year per person.
Reform said Justice Secretary David Lammy has written to "MPs claiming he has strengthened the government's ability to deport foreign criminals."
But Reform said: "However, he also says in the same letter that 'this is subject to several exceptions, including where to do so would be a breach of a person's ECHR rights or the UK's obligations under the Refugee Convention'. So, in other words nothing would change and no one would be deported."
The Sentencing Bill, which aims to ease the crisis in prison places, will restrict the use of short sentences and strengthen community punishments.
It will also include an "earned progression scheme", which allows convicts who demonstrate good behaviour to be freed earlier.
The measures will allow prisoners on standard determinate sentences who obey prison rules to be released early, with enhanced supervision in the community, followed by a period on licence, which is unsupervised.
There will be a minimum release point of 33% for standard determinate sentences, while more serious standard determinate sentences will have a 50% minimum.
This will be accompanied by an expansion of tagging to monitor offenders in the community.
Labour have said the worst offenders - those on extended determinate sentences or life sentences - will never benefit from early progression.
The Sentencing Bill will implement the recommendations from the review of the system carried out by Tory former justice secretary David Gauke.
The "earned progression scheme", recommended by Mr Gauke, would see an offender's release date determined by their behaviour. If they follow prison rules, they will earn early release. If they do not, they will be locked up for longer.
Reform UK warned: "This means that if it passes unamended, dangerous criminals locked up for some of the worst possible offences including paedophiles could be let out back into the community after serving only a third of their sentence behind bars."
The Express has revealed how 2,000 perverts convicted of child sex offenders will benefit for softer sentences.
Analysis of Ministry of Justice sentencing statistics revealed 2,254 offenders were locked up for child sex offences last year.
This included 167 perverts jailed for raping a female under 16.
Some 548 paedophiles jailed for taking, permitting, making, distributing or publishing indecent images of children will also spend less time behind bars.
And 229 people convicted of sexual grooming could be allowed home sooner, figures show, if a similar number is jailed under the proposed new 'progression model'.
Another 200 offenders - aged over 18 - jailed for sexual activity involving a child under 16 - will spend less time in prison.
A staggering 104 people jailed for sexual activity with a child under 13, and 248 caged for sexually assaulting a girl under 13 could also benefit from the softest sentencing reforms in a generation.
A shocking 54 people locked up for raping a child under the age of 13 could also be let out earlier.
This means a staggering 82% of offenders convicted of child sex offences will receive a lighter sentence, when compared against last year's figures.
The Daily Express on Monday revealed the "progression model" - which could allow offenders out after serving just 33% of their sentences - will apply retrospectively.
This means thousands already locked up for crimes such as rape, sexual assault, burglary, stalking, domestic abuse and even death by dangerous driving will serve less time - despite already being sentenced.
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